Certain substances, when bound to the membranes of neurons, cardiac and skeletal muscle, salivary acini, and other cells, behave as molecular indicators of membrane potential. The optical properties of these molecules, most notably fluorescence and absorbance, vary in a linear fashion with potential and may, therefore, be used to monitor action potentials, synaptic potentials, or other changes in membrane voltage from a large number of sites at once, without the necessity of using electrodes. We propose to develop more sensitive probes, to extend the technology associated with their use, and to employ these molecular voltmeters for optical recording of membrane potential from hitherto inaccessible regions of single neurons such as axon an neuroendocrine terminals and axonal and dendritic processes and from many sites simultaneously in small assemblages of neurons and in electrical syncitia, in order to study the spatial and temporal patterning of activity. First, we intend to use a computer based system for Multiple Site Optical Recording of Transmembrane Voltage (MSORTV), already constructed and capable of monitoring changes in membrane potential from as many as 124 loci at once, to record patterns of electrical activity throughout syncitia (such as glandular tissue), and to study the properties of truly simple nervous systems -- small artificially constructed ensembles of synaptically connected invertebrate central neurons maintained in culture -- by recording electrical activiy optically from all of their components simultaneously. Second, we propose to use this appratus, with an Argon ion laser light source, to record membrane potential changes from fine processes of single neurons in situ, within an invertebrate neuropil, and isolated in tissue culture. These structures are not penetrable by microelectrodes and are frequently too far away, electrically, for their activity to be reflected in the somata. Finally, we expect to exploit the optical properties of potentiometric probes, and our multiple site optical recording capability, to detect potential changes in vertebrate nerve terminals, and to correlate alterations in the shape of the nerve terminal action potential with the release of neuropeptides.